usb: gadget: g_dnl: Fix NULLPTR dereference when serial# is unset

The current behaviour of this function will dereference a null pointer
if the serial# environment variable is unset. This was discovered on a
board where U-Boot did not have access to the first 256MB of ram,
resulting in a board crash.
In the event that U-Boot has full access to memory, it will still read
from address 0, which is probably not optimal.
This simple check is enough to fix it

Signed-off-by: Michael Ferolito <[email protected]>
Cc: Marek Vasut <[email protected]>
Cc: Heiko Schocher <[email protected]>
Cc: Kyungmin Park <[email protected]>
Reviewed-by: Heiko Schocher <[email protected]>
Reviewed-by: Mattijs Korpershoek <[email protected]>
Reviewed-by: Marek Vasut <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Mattijs Korpershoek <[email protected]>

0 files changed, 0 insertions, 0 deletions